I will put the Standard Model math explanation series on hold for a bit and in subsequent posts I'll explain this result. ALL of quantum mechanics formalism follows from those 4+1 physical principles in a rigorous, constructive, step by step argument. Both the Hilbert space and the state space realizations are derived.

The axioms are minimal:

- Composition (tensorial, categorical) arguments are needed because there are classical physics models for quantum mechanics for a single particle. Correlations between systems are the essential quantum characteristic.

- Information theoretical arguments (positivity) are used because composition arguments produce a third unphysical solution. However there is no "it from bit", but: "it is what can generate a bit".

Friday, July 25, 2014

Cartan structural equations

Elie Cartan was a French mathematician who made fundamental contributions in differential geometry and group theory.

Today I want to continue the discussion regarding curvature and talk about Cartan's modern approach to differential geometry. Cartan died in 1951, three years before Yang and Mills wrote their seminal paper on gauge theory and he did not see how his ideas are used by Nature in all the four fundamental interactions: gravity, electromagnetism, weak force, and strong force.

Last time I presented the Christoffel symbols and I noted that they depend on the coordinate system used. Riemann uncovered an intrinsic geometric object called Riemann curvature tensor which expressed in terms of the Cristoffel symbols is:

and compute \( d \omega^\delta_\gamma \) and \( \omega^\delta_\mu \wedge \omega^\mu_\gamma \) and compare the sum with the curvature equation in terms of \( \Gamma \) we get Cartan's local structural equation:

Now those two equations are a thing of maximal physical beauty. Sure, they look alien and vaguely interesting in this abstract mathematical form which Cartan derived, but inside them hides Maxwell's equations, Yang-Mills gauge theory, and Einstein's general relativity. In the next posts it will be our job to extract the physics from them.

For example if \( A \) is the electromagnetic 4-potential because the electromagnetic gauge theory is abelian, \( A \wedge A = 0 \) and the two equations from above are nothing but Maxwell's equations as we will show later. Please stay tuned.

Friday, July 18, 2014

Christoffel symbols

Gauge theory has its roots in general relativity, and we have to start there if we want to properly understand it. (Also general relativity will become our friend and we will come back to it to compare ideas from gauge theory.) An excellent book to learn general relativity is Gravitation and I was very fortunate to learn it as a graduate student directly from Professor Misner. The book's famous saying is: "spacetime tells matter how to move; matter tells spacetime how to curve".

What does it mean spacetime curves? Here I recall an explanation from Gravitation which made a strong impression on me at the time. If you throw a ball you see its curved trajectory as it is attracted by Earth. Similarly, a bullet's trajectory is also curved down, but by a very tiny amount. Why is this huge discrepancy in the amount of curvature in each case? Naively one may say that because of the speed, but how about the equivalence principle? The mass should make no difference in what it basically amounts to a free fall motion where the gravitational effects vanish along the trajectory in an "Einstein's elevator". Therefore one would expect that the curvature is the same!!! And it is the same! Only if one thinks in 3+1 spacetime dimensions, not in 3 space dimensions. Here is the picture from page 33 of the book which explains it all:

Space-time, or in general any Riemannian space is characterized by a metric tensor. When the space is curved, the ordinary derivative has to be modified to take into account changes to the local metric. The corrected derivation operation is called covariant derivative. The difference between the ordinary derivative \( \partial \) and the covariant derivative \( D \) is the Christoffel symbol \( \Gamma \):

\( \Gamma \) is also called the affine connection because it helps define the parallel transport along geodesic lines which locally are the straight lines in the curved space ("spacetime tells matter how to move"). Along geodesics, the covariant derivative \( D \) is zero. Gravity does not bend light, There is no dynamical explanation of light bending, but a kinematic one. Light always continues to move locally in a straight line, but specetime itself is warped by the presence of mass. The geodesics are affected by mass. If you ever traveled in an airplane, the trajectory from the starting point to the destination appears curved on the TV screens in the plane. The plane always goes straight from point A to point B on the shortest path to save fuel, but the trajectory appears curved because Earth is not flat, but a curved space (a sphere). The apparent trajectory curvature is encoded by the Christoffel symbols.

So what is then the covariant derivative of a more complex object like say a tensor \( T^{\alpha \beta}\)?

This formula is easily generalized in case of an arbitrary tensor of n indexes: \( \Gamma\) appears n times there. We can apply this to the metric tensor \( g_{\mu \nu}\) and after some algebraic manipulations solve for \( \Gamma \):

The moral of the story is that on Riemann spaces, because the Christoffel symbols depend on the metric, the covariant derivative depends on the metric as well and everything is defined using intrinsic notions. There is only one "fly in the ointment": \( \Gamma \) itself is not a tensor. Why? Because it depends on the partial derivative of the metric tensor with respect of the coordinate system. Change the coordinate system and you get a different \( \Gamma \). What we need is a better mathematical object which changes nicely when the coordinate system is changed. In other words, we need the curvature tensor.

Next time we'll introduce the curvature tensor and we'll talk about a key physical principle: curvature is force.

Friday, July 11, 2014

Short Exact Sequences

We have slowly introduced the tools needed to understand modern physics and we are almost there before seeing them in action. One particular mathematical pattern proves to be very useful and is one of those cases where you need to recognize at first sight: a short exact sequence.

We have seen exact sequences before in the homology and cohomology posts where a set of groups \( G_1,...,G_n \) are linked by maps and the image of one map is the kernel of the next map. Then you can only "hop twice" an element through the sequence before you end up in the identity element of the group. The geometric interpretation is that "the boundary of a boundary is zero".

So what happens when the exact sequence is short? The best way to get this is to work the problem from the other end. Suppose we have two sets A and C (I am skipping B on purpose). We can then construct the Cartesian productAxC and for each element from AxC we can understand A as an equivalence class. Then C ~ AxC/A and we can introduce a short exact sequence:

Let's first clarify what is with the zeros at each end. The first zero means that the map between A and AxC is injective (one-to-one). This is because the first zero can only be mapped to the zero element of A and that in turn must be the only element which maps to the zero element of AxC. Similarly the last zero element implies that the map between AxC and C is surjective (onto).

The second map is the reduction modulo 2: for any \( m \in Z, m=2k+p\) we take k=0.

So is C ~ B/A: Z/2Z = {0,1} ~ Z/Z?

Z/Z = {m in Z | m=0} = {0} so the answer is no.

In general knowledge of A and C does not determine B. This is because A and C can interlock in a nontrivial way and B is not necessarily AxC!
So how about some fancy short exact sequences:

\( 0 \rightarrow S_1 \rightarrow S_3 \rightarrow S_2 \rightarrow 0 \)

\( 0 \rightarrow S_3 \rightarrow S_7 \rightarrow S_4 \rightarrow 0 \)

Those are two of the celebrated Hopf fibrations. I the first case, S3 is a sphere in 4 dimensions and it can be decomposed into an S2 (the ordinary sphere in 3 dimensions) where in each point you have a circle (S1). The picture at the top of this post is a stereographic projection of the rigamarole S3 Hopf fibration because S3 cannot be plotted directly.

The real interesting use of short exact sequences however occurs in physics in gauge theory in fiber bundles.

In particular Connes' non-commutative geometry model of the Standard Model is best understood if you work out first a simpler unphysical proposal which gives rise to a short exact sequence. We'll get there...

Friday, July 4, 2014

Exact, Coexact and Harmonic (Hodge Theory)

Happy 4th of July

Today, in celebration of Independence day we'll have some mathematical fireworks :)

It is customary to learned in school about the dot product and the cross product. The dot product comes from projecting one vector onto the other, while the
cross product creates a new (pseudo) vector out of two other vectors. The cross
product is basically a historical accident which got accepted on due to its practical
convenience but a better concept is the exterior product. Even better we can understand all of this in the framework of Clifford algebras.

Here is how it goes. We’ll work out the usual 3D space for
convenience. Start with the 3 x,y,z unit vectors and call them: \( e_1, e_2,
e_3\). Then introduce 2 practical rules:

\( e_1 e_1 = e_2 e_2 = e_3 e_3 = 1 \)

\( e_i e_j = - e_j e_i \) when \( i \ne j \)

Think of the unit vectors as matrices which collapse to the identity when multiplied by themselves, and anti-commutes.

So why are we looking at this compact formalism for Maxwell's equations? Because electromagnetism is only one of the 4 fundamental forces in the universe: gravity, weak force, electromagnetism, strong force, and the weak and strong forces are described by Yang-Mills gauge theory which is a generalization of Maxwell's theory. Without a compact notation and a clear geometrical meaning, we have no hope of understanding Yang-Mills theory and we will be stuck forever in the La La land of using cross products, gradients, divergences, and equations in components.